Tributyltin catalytic

The palladium-catalyzed hydrostannylative cyclization of enynes is dealt with first, since mechanistically it is closely related to hydrometallation. Lautens262 reported the formation of homoallyl stannanes through the reaction of 1,6-enynes with tributyltin hydride in the presence of a catalytic amount of Pd(OAc)2.263 The active catalytic species is... 

Like most aryl halides, furyl halides and furyl triflates have been coupled with a variety of organostannanes including alkenyl, aryl, and heteroaryl stannanes in the presence of catalytic palladium. Carbamoylstannane 66 was prepared by treating lithiated piperidine with carbon monoxide and tributyltin chloride sequentially. The Stille reaction of 66 and 3-bromofuran then gave rise to amide 67 [61]. In another example, lithiation of 4,4-dimethyl-2-oxazoline followed by quenching with MesSnCl resulted in 2-(tributylstannyl)-4,4-dimethyl-2-oxazoline (68) in 70-80% yield [62], Subsequent Stille reaction of 68 with 3-bromofuran afforded 2-(3 -furyl)-4,4-dimethyl-2-oxazoline (69). 

A novel homolytic substitution yielded 2-(tributylstannyl)benzothiazole [35]. Thus, 2-(alkylsulfonyl)benzothiazole 49 was allowed to react with 2 equivalents of tributyltin hydride in the presence of catalytic azobisisobutyronitrile (AIBN) in refluxing benzene, affording 2-(tributylstannyl)benzothiazole along with tributylstannylsulfinate 50. 

Table 9.6 shows the effect of both the addition time and the polarity of the solvent, as well as the nature of the aldehyde, in the catalytic asymmetric aldol condensation promoted by tributyltin fluoride and a chiral diamine coordinated to tin(n) triflate. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

Sibi s group studied a similar reaction using ligands 9b, 34a-c, and 161 with iodides 157b and 157c, tributyltin hydride 160 and A-crotonyl oxazolidinone 80a or A-cinnamoyl oxazolidinone 80b.As shown in Table 9.29 (entries 7 and 9), the inda-box ligands exhibited optimum results with yields up to 92% and selectivities up to 93% (ee). The use of the ligand-metal complexes in catalytic amounts led to lower yields and enantioselectivities (Fig. 9.50). ... 

The thionoimidazolide 1, tributyltin hydride and a catalytic amount of AIBN are taken up in benzene and the mixture is warmed to reflux which provides a mixture of the bicyclic products yield 68% d.r. (endo/exo) 73 27. 

Representative examples are listed in Table 11. Further examples are reported in refs 215 — 217. Similar ring enlargements were found for l-(3-halopropyl)-2-methylenecyclobutanes under the same reaction conditions.218 Thus, 4 is converted to 5 in refluxing benzene after slow addition of tributyltin hydride and catalytic amounts of 2,2 -azobisisobutyronitrile over a period of several hours. Further examples of this reaction are given in ref 218. 

To 244 mg (0.64 mmol) of the starting bromide (E/Z ratio 1 7) in dry toluene (0.015 M) under reflux was added 2.4 eq of tributyltin hydride and AIBN (catalytic amount) in 3 h through a syringe pump. The reaction mixture was cooled and the solvent was evaporated. The residue was dissolved in ether and 10% aqueous KF solution was added, and the mixture was stirred for 18 h. The organic phase was separated, dried, and evaporated. After flash chromatography (hexane-ethyl acetate, 90 10) of the residue gave the 134 mg (80%) of the product mp 75-77°C [a]D —61° (c 1.2, CHC13). Minor amounts of the noncyclized reduction product and the isomeric compound with an a-CHjCOjMe were also isolated. 

Catalytic procedures (introduced by Kuivila and Menapace92) are easier to conduct and the tin hydride concentration is more easily controlled. A catalytic amount of tributyltin hydride or tributyltin chloride is mixed with the radical precursor, the alkene acceptor and a stoichiometric quantity of a coreductant such as sodium borohydride93 or sodium cyanoborohydride.29 Over the course of the reaction, the borohydride continuously converts the tin halide to tin hydride. The use of the catalytic procedure is probably restricted to halide precursors (tin products derived from other precursors may not be reduced to tin hydrides). This method has several advantages over the standard procedures (i) it is simple to conduct (ii) most functional groups are stable to the coreductants (especially sodium cyanoborohydride) (iii) the tin hydride concentration is known, is stationary (assuming that the tin halide is rapidly reduced to tin hydride), and can be varied by either changing the concentration of the reaction or the quantity of the tin reagent (10% is a typical value, but lower quantities can be used) and finally, (iv) the amount of tin hydride precursor that is added limits the amount of tin by-product that must be removed at the end of the reaction. 

Yamamoto has recently described a novel catalytic, asymmetric aldol addition reaction of enol stannanes 19 and 21 with aldehydes (Eqs. 8B2.6 and 8B2.7) [14]. The stannyl ketones are prepared solvent-free by treatment of the corresponding enol acetates with tributyltin methoxide. Although, in general, these enolates are known to exist as mixtures of C- and 0-bound tautomers, it is reported that the mixture may be utilized in the catalytic process. The complexes Yamamoto utilized in this unprecedented process are noteworthy in their novelty as catalysts for catalytic C-C bond-forming reactions. The active complex is generated upon treatment of Ag(OTf) with (R)-BINAP in THF. Under optimal conditions, 10 mol % catalyst 20 effects the addition of enol stannanes with benzaldehyde, hydrocinnamaldehyde, or cinnamaldehyde to give the adducts of acetone, rerf-butyl methyl ketone (pinacolone), and acetophenone in good yields and 41-95% ee (Table 8B2.3). 

Given the selection of an appropriate substrate, the generation of a carbon centred radical from such a substrate can initiate a series of bond-making and bond-breaking processes which are sometimes referred to as radical cascade reactions. These can be of great synthetic value. Thus, treatment of the ketene dithioacetal 1 with a five fold excess of tributyltin hydride in hot benzene under nitrogen and in the presence of a catalytic amount of AIBN gave the metallated benzo[ fc]thiophene 2, itself a valuable synthetic intermediate, in 70% yield. 

Generating tin hydride in situ by using a catalytic amount of tributyltin chloride and sodium cyanoborohydride [24] did not solve the problem. Although this method inherently produces the desired low concentration of n-BujSnH to promote cyclization over direct reduction of the starting material and reduces drastically the amount of tin residue, a displacement reaction between solvent and the benzylic hydroxyl group takes place. Eventually, the best method was the use of high dilution conditions at approximately 70 °C in benzene followed by the KF workup. 

The treatment of 1,2,3-selenadiazoles (79) with a catalytic amount of tributyltin hydride and AIBN in the presence of an excess of olefin, gives 2,3-dihydroselen-ophenes (80) in good yield (Scheme 20). The reaction proceeds via tributyltin radical-promoted denitrogenation to give a vinyl radical, which then adds to the olefin followed by intramolecular cyclization. Under similar conditions 1,2,3-selenadiazoles and aliphatic or aromatic ketones afford alkynes as the sole products [114, 115]... 

With many heterocycles as substrates, radical substitutions can be successfully carried out under the conditions of phase-transfer Gomberg-Bachmann reactions [145]. Again, as above, mainly aryl and heteroaryl halides have recently been used as precursors for heterobiaryl compounds. A study including the arylation of a large number of heterocycles in the presence of tributyltin hydride and catalytic benze-neselenol led to the results that mostly biaryl products are obtained from nitrogen-containing heterocycles, whereas furan and thiophene gave partially dearomatized compounds [155]. 

Tsai and coworkers89,91,246,247 reported the synthesis of cyclic silyl enol ethers and silyl ethers by using a radical cyclization followed by the radical Brook rearrangement (equation 111). The cyclization of 4-bromo-4-stannylbutyl silyl ketones 188 in benzene with a catalytic amount of tributyltin hydride and AIBN gave cyclic silyl enol ethers 18989 91 247. The whole catalytic cycle proposed is shown in equation 112. 

The ruthenium allenylidene complexes W are excellent precursors for the catalytic dimerization of tributyltin hydride under mild conditions [ 109] (Eq. 15). In the presence of Bu3SnH, the hydride addition at Cy provides a catalytically active alkynyl ruthenium-tin species (Scheme 22). 

The diastereoselective aldol reaction of the tributyltin enolate of cyclohexanone with benzaldehyde in the presence of a catalytic amount of various metal triflates has been studied. The highest A-selectivity is observed with Pd(OTf)2, while Zn(OTf)2 in THF shows moderate yy -selectivity (Equation (68)).222 OSnBu3... 

A complementary sequence uses an alkyl halide 7.46 with a Z-substituent to create an electrophilic radical 7.47 in the presence of a nucleophilic alkene 7.48. In this case, the radical 7.49 expels the low-energy tributyltin radical to regenerate the tin radical achieving overall the allylation of the ester, catalytic in both the AIBN and the tin hydride. The Z-substituent in the radical 7.47 is necessary for an efficient reaction—in its absence the allylstannane has to be used in large excess. 

The first palladium-catalyzed formation of aryl C-N bonds was reported in 1983 by Migita and co-workers, Eq. (1) [14], The reaction of electronically neutral aryl bromides and aminotin compounds in the presence of catalytic [(o-tol)3P]2PdCl2 resulted in the efficient preparation of the corresponding aniline in moderate to good yield. This seminal discovery was limited by the necessity to use the thermally and moisture sensitive tributyltin amides, however. 

---